Once upon a time, Earth was a barren place. Everything changed when something went wrong with the chemistry that existed at the beginning of our planet’s history; recycling existing matter to survive, reproduce, and thrive. What this something was, and when it first wriggled, have probably been burning questions that have puzzled humanity for as long as we’ve been able to ask, “What am I?” Now a new study has found some answers, and life has emerged surprisingly early.

By studying the genomes of organisms living today, scientists have determined that the Last Universal Common Ancestor (LUCA), the first organism to give rise to all life on Earth today, emerged as early as 4.2 billion years ago. In Earth terms, that’s about 4.5 billion years. This means that life first emerged when the planet was still young.

“We did not expect LUCA to be this old, just hundreds of millions of years before the Earth formed,” says evolutionary biologist Sandra Alvarez-Carretero of the University of Bristol in the United Kingdom. “But our results are consistent with current views of the habitability of the early Earth.”

When the Earth was new, it was a very different place, with an atmosphere that we now know to be extremely toxic. The oxygen that seems necessary for modern life only appeared relatively late in the planet’s evolutionary history, about 3 billion years ago.

But life emerged earlier; we have fossil microbes from 3.48 billion years ago. Scientists believe that conditions on Earth were stable enough to support life around 4.3 billion years ago. But our planet is subject to erosion, geological and organic processes that make it nearly impossible to find evidence of life from that period. So a team of scientists led by phylogeneticist Edmund Moody of the University of Bristol went looking elsewhere: the genomes of living organisms and fossils.

Their research is based on something called the molecular clock. Essentially, we can estimate the rate at which mutations occur and count the number to determine how much time has passed since the organisms in question diverged from their common ancestor.

All organisms, from the simplest microbe to the strongest fungus, have some common characteristics. There is a universal genetic code. The method of protein production is the same. There is an almost universal sequence of 20 amino acids oriented in the same direction. And all living organisms use adenosine triphosphate (ATP) as a source of energy in their cells.

Based on these similarities and differences, Moody and his colleagues were able to determine how much time had passed since LUCA’s successors began to diverge. And using advanced evolutionary modeling, they were able to learn more about LUCA itself, what it was, and how it survived on an Earth so inhospitable to its descendants.

They found that LUCA was likely very similar to a prokaryote, a single-celled organism that lacks a nucleus. It appeared to be oxygen-insufficient, so it was not dependent on oxygen—not unexpected for a microbe. So its metabolic processes probably produced acetate. But there was something else interesting. It turns out LUCA wasn’t alone.

“Our study shows that LUCA was a complex organism, not very different from modern prokaryotes,” says David Pisani, a phylogenomics expert at the University of Bristol.

“But what’s really interesting is that it clearly had an early immune system, suggesting that our ancestor was engaged in an arms race with viruses as early as 4.2 billion years ago.”

It may have emerged shortly after LUCA, as its metabolic processes produce waste that other life forms can use. This means that it takes relatively little time in the evolutionary history of a planet for a complete ecosystem to emerge—a discovery with implications far beyond our little pale blue dot.

“Our study brings together data and methods from many disciplines to reveal insights into the early Earth and life that are not available from any single discipline,” explains Philip Donoghue, a palaeobiologist at the University of Bristol.

“This also shows how quickly ecosystems were established on early Earth. This suggests that life could have developed in terrestrial biospheres elsewhere in the universe.” The study was published in: Nature Ecology and Evolution.